The present invention relates to a locking device which may be employed as a lock proper, i.e. directly locking a door, a drawer in a piece of furniture, or a strong box, or may serve to additionally safeguard an existing, conventional lock against being tampered with. More particularly the device forming the subject of this patent application is of the type which is operated by the insertion of a card like a small metal or plastic plate (to be referred to in this specification and the claims as "card").